1. Field of the Invention
The invention relates to caulking guns and to dispensing cartridges, and more particularly to the type of composition dispensers in which a cartridge is placed into a gun structure and a piston urges a plunger forwardly from the rear of the cartridge, thus reducing a volume available for the composition inside the cartridge and forcing the composition from an open tip at the front of the cartridge.
These types of caulking guns have been the subject of undesireable afterflow, i.e. the interior of the cartridge is still subject to overpressure after the plunger is no longer actively urged forward and, as a result, additional amounts of composition are forced from the cartridge.
Two primary reasons for the afterflow phenomenon are recognized. Firstly, the usually thin-walled cartridge expands during the plunger actuation and, according to the physical law that systems always attempt to return to the relaxed state, the cartridge wall relaxes after the plunger actuation. Due to the fact that prior art backwalls of the cartridges have been devised to retain their forward-most position and that the plunger of the caulking gun is typically locked against a return movement, the relaxation of the cartridge wall leads to afterflow, i.e. to oozing at the dispensing tip. Secondly, most caulking compositions have a high degree of viscosity and are at least marginally compressible, which, upon plunger actuation, causes a substantial internal pressure buildup which, after the plunger is no longer forced forward, also leads to oozing at the dispensing tip.
2. Description of the Related Art
The afore-described afterflow problem is often answered in the context of conventional prior art structures by quickly releasing and moving back the gun plunger as soon a sufficient amount of composition has been dispensed.
U.S. Pat. No. 5,236,105 to Galex describes a novel system for preventing over-ejection. In that system, conventional caulking guns are retrofitted with several members, namely a female element, male element, a return spring, and a stop. The spring is utilized as an active biassing element which actively pulls back the backwall in the cartridge and thus introduces a relative vacuum inside the cartridge.
U.S. Pat. No. 4,834,268 to Keller describes a plunger system in which an elastic sealing ring is urged towards the inner wall surface of the cartridge by a radial component of the force which urged to plunger forwardly in the dispensing mode. When the plunger is no longer actuated, the sealing ring relaxes slightly and allows the plunger to relax the inside cartridge pressure.
The first of the above-noted methods of preventing afterflow is clearly unsatisfactory. The systems described in the two afore-mentioned patents are quite complicated and thus rather expensive.